I’ve capitalized SUBJECTS and VERBS in your sentences to make things clearer.
Remember that suus-a-um REFERS to the SUBJECT of the sentence but DESCRIBES not the subject but SOMETHING BELONGING TO THE SUBJECT.
And remember that suus-a-um is an adjective (so its ending must agree with that of a noun/pronoun) while eius is a pronoun (= his/her/its) and doesn’t change.
Ubi est pecunia Iulii? PECUNIA suum in sacculo EST.
(Incorrect on 2 counts: First, the adjective ‘suUM’ doesn’t agree with anything else in your sentence. Second, changing ‘suum’ to ‘sua’ wouldn’t help because PECUNIA is the subject; it’s not the money’s money we’re talking about. It’s Julius’s. HIS money. That is, ‘EIUS pecunia’ (since he’s not even in the sentence he can’t be its grammatical subject).
IULIUS pecuniam suam in mensa PONIT.
(Correct: ‘suam’refers to subject-of-sentence Iulius and agrees with pecuniAM, something belonging to him)
AEMILIA pecuniam eius VIDET.
(Correct: it’s his (Julius’s) money that Aemilia sees. IF it were her own money, it would be suam, referring to her (the subject) and agreeing with pecuniAM, the thing ‘owned’)
Adestne Servus Iulii? SERVUS suum ADEST.
(This is a no-no. Apart from the fact that once again ‘suum’ doesn’t agree with anything else in your sentence, changing ‘suum’ to ‘suus’ wouldn’t help either since suus-a-um must refer to something ‘belonging to’ the subject. What would that be? Julius (outside the sentence) is the ‘owner’ and ‘his’ must refer to him. So we need ‘eius’.
IULIUS servum suum VIDET.
DAVUS venit et dominum suum SALUTAT.
DAVUS sacculum suum in mensa PONIT.
In sacculo suo PECUNIA non EST:
(Incorrect: Subject is pecunia. Technically could be correct (after all ‘Cleopatra est in foro suo’ - ‘Cleo is in her forum’ would be correct) but although ‘The money’s in its bag’ might sound OK in English the ‘owner’ in this context is obviously some person or other: The money’s not in HIS/HER bag. Eius.)
SACCULUS suum vacuus EST.
(Incorrect: Wrong for 2 reasons as above. Got it?)
DAVUS nummal (=nullam?) pecuniam in sacculo suo HABET.
(Correct apart from typo)
IULIUS unim (=unum) nummum in sacculo eius PONIT.
(Correct if the bag is Davus’s, incorrect if the bag is Julius’s.)
Iulius: "Sume sacculum tuum!" DAVUS sacculum suum SUMIT.
MEDUS non suum sed Iulii in sacculo suo HABET
(Something missing here ...)
By the way, note for future reference that in ‘Iulius nummum IN sacculO PONIT’ it’s IN sacculO (ablative) and not IN sacculUM (accusative) which we as English speakers might expect because ‘put’ usually implies motion in our minds and Latin grammar says IN is normally followed by the accusative when motion is implied. The verb ‘ponere’ (to put) is a bit special in this respect. Likewise, ‘Davus sacculum suum IN mensA (ablative) PONIT (not ‘IN mensAM (accusativ) PONIT’ even though for us ‘put on/onto the table’ implies motion.